Variable angle reflectance is an important spectroscopic technique. Certain samples, such as opaque substances, films on opaque substrates, and films on liquids, are tedious or practically impossible to analyze with conventional transmission spectroscopy equipment. The analyses of such samples by reflection spectroscopy, however, are straightforward.
There are three different methods of reflection spectroscopy. The technique employed depends on the sample nature and the information sought. External reflection spectroscopy is used with thin films on opaque substrates and opaque smooth solids. Internal reflectance permits spectral measurements of liquids, powders, pastes, gels, and soft solids. Diffuse reflectance is most commonly used for the analysis of powders and rough surface solids. Variable angle studies, using either external or internal reflection spectroscopy, provide data for the determination of optical constants and sample thicknesses. For further background on the state of the art of this technology, reference is had to U.S. Pat. No. 3,603,690, and the Harrick Scientific Corporation catalog HSC-831 which describes a number of the currently available reflection accessories.
In our earlier-issued patent, U.S. Pat. No. 5,048,970 ('970), the contents of which are hereby incorporated by reference, we describe and claim a variable angle reflection accessory for use in reflectance spectroscopy which offers a number of important benefits in the field, such as the ability to vary the angle of incidence on the sample over a very wide range while maintaining optical alignment and continuing to center the radiation beam on the same sample area, and a rotation mechanism providing a linear relationship between user rotation of a knob and the resultant change in the angle of incidence. In this accessory, the sample holder is provided inside the accessory interior.
In our copending application, Ser. No. 07/831,529, now U.S. Pat. No. 5,177,561, the contents of which are hereby incorporated by reference, we describe a novel accessory construction for use in the sampling compartment of conventional spectrometers that allows the accessory to be coupled directly to the purge system of the spectrometer, so that the entire optical path lies in an absorption-free atmosphere. With certain accessories, where the sample is also mounted external to or on the outside of the accessory housing, this can offer the further advantage that samples are easily exchanged without breaking the purge seal thereby greatly increasing sample analysis throughput. This is important for certain users. This benefit, however, cannot be obtained with the variable angle reflectance accessory described in the '970 patent because of the location of the sample holder inside the accessory housing.